I have a Coleman natural gas furnace. I diagnosed a bad gas vavle (24v from board, gas vavle didn't open) i replaced the gas vavle it worked for a week and then did the same thing. I Cleaned the burners, orifies and flame sensor. And replaced the gas vavle a 2nd time, still doesn't work. Gas has been bled off at the drip leg also so i know there is gas. Any ideas?